I'm not sure I have just one.
If I had to pick, I'd say it's because the series surprises me, that it's unpredictable. Not that you can't see some things coming, but for the most part, there are so many things that shocked me the first time I read through the books. I don't find a lot of books that do that, and I really love and appreciate when they do.
Other than that, I think Martin does a wonderful job with making the story easy to read and accessible while keeping the plot so complex, and I love that his characters are never one-dimensional.
You said above you haven't read them yet? They are wonderful. I bought the first one off eBay, and by halfway though, I ordered the other three off Amazon, and I finished up days before the books arrived. It was so hard to wait.
I'm lucky the fifth book is coming out so soon after I started the series, so the wait hasn't been as long for me as a lot of other fans, but there are still two books left to be written, so I'm sure it will get to me (though I am used to waiting, as I've been faithfully waiting for the third book of Melanie Rawn's Exiles series since 2001).